Continuous steelmaking process

ABSTRACT

A process for the manufacture of steel in which a charge material comprising direct reduced iron, and optionally containing steel scrap, is continuously charged into a stationary electric arc furnace. Inside the furnace, a bath of molten steel and a continuous slag layer are maintained, the steel bath preferably having a volume of approximately 7 to 13 tap volumes. Heat for melting the charge material is preferably provided by open arcs between the electrodes and the metal bath, with the slag layer preferably being superheated to about 40 to 150° C. above the bath temperature. In order to prevent excessive superheating, the slag preferably has a melting point substantially the same as, or greater than, the bath temperature, with the slag melting temperature being adjustable by varying the amount of MgO in the slag. The slag preferably has low basicity to reduce the rate of refractory erosion. The steelmaking process is preferably carried out in a steel manufacturing plant in which a direct reduction furnace is closely coupled to the electric arc furnace. Preferably, the direct reduction furnace is “stacked” above the electric arc furnace so that the direct reduced iron can be fed to the electric arc furnace by gravity through inclined conduits.

FIELD OF THE INVENTION

The present invention to relates to steelmaking, and more particularlyto continuous processes for steelmaking.

BACKGROUND OF THE INVENTION

A challenge facing the steel industry in its never-ending quest forgreater efficiency in the development of a continuous steelmakingprocess. While the upstream iron making and the downstream casting ofsteel are well-established continuous processes, steelmaking remainsessentially a batch operation.

Steelmaking is typically conducted in a circular electric arc furnace.Reduced iron in the form of small spheres or briquets, and optionallysome steel scrap, is charged into the furnace and melted by very highpower electrodes, creating a power density in the furnace hearth ofabout 2000 to 2500 kW/m². The molten metal is protected and partlyrefined by a liquid slag layer comprised primarily of metal oxides.After the charge is melted, the molten metal is poured or tapped fromthe furnace for subsequent alloying and casting.

A number of processes for continuous steelmaking have been proposed inthe prior art. However, none have proved to be completely satisfactory.

One example of a continuous steelmaking process is proposed in U.S. Pat.No. 4,119,454 (Rath). According to the Rath patent, the steelmaking stepis performed continuously in a stationary round or rectangular electricarc furnace. The fu mace electrodes are immersed in the slag layer,which acts as a resistive heating element to heat the underlying moltenmetal layer. In the Rath patent, scrap steel is fed continuously orintermittently into the furnace through one or more openings, andtapping of slag and steel is performed periodically. The metal bath inthe furnace is maintained at a volume of about 1 to 10 tap volumes.

The Rath process uses a highly reactive and superheated slag, with theoperating temperature of the slag being 40-100° C. above the steelmelting temperature and 70-220° C. above the slag melting temperature.Thus, the melting temperature of the slag used in the Rath process isabout 30-120° C. less than the steel melting temperature. Thissuperheated slag has very high liquidity and high reactivity, andresults in excessive wear on the furnace refractory.

U.S. Pat. No. 4,133,468 (Gudenau et al.) describes a method forcontinuously melting sponge iron (also referred to herein as “directreduced iron” or “DRI”) in the production of steel having a carboncontent of as low as 0.015%. According to Gudenau et al, the furnaceelectrodes are immersed in a foaming slag layer. The slag is basic, hasa CaO/SiO₂ ratio sufficient to maintain good liquidity, and containsfrom 7 to 30% FeO and 5 to 12% MgO. A slag according to this compositionmay, however, still have high fluidity and high reactivity with respectto the furnace refractory.

U.S. Pat. No. 3,463,269 (Hatch) describes the use of a stationarysix-electrode rectangular electric arc furnace in the production ofsteel. The furnace is charged continuously with sponge iron to producethe final or semi-finished steel. Steel scrap is charged into thefurnace at the beginning of the furnace campaign in order to form a pool(“heel”) of molten steel which remains in the furnace below the tap holeat all times. In the Hatch process, the power input is limited bymaintaining the metal bath temperature just above the steel meltingpoint to minimize under-cutting of the slag and refractory attack, andto prevent violent boiling due to reaction of carbon with metals in theslag. This method has a number of disadvantages, including thepossibility that the furnace will be run with a highly superheated slagof high basicity. Furthermore, the Hatch method does not disclosecontinuous charging of scrap into the furnace.

Therefore, there is a continued need for a continuous method ofsteelmaking.

SUMMARY OF THE INVENTION

The present invention overcomes the above-described disadvantages of theprior art, by providing a continuous steelmaking process in which ironis continuously fed to and melted in a stationary electric arc furnace,with refractory wear being minimized through the use of a relativelyhighly basic, high melting slag composition.

In one aspect, the present invention provides a slag composition for usein a steel manufacturing process in which iron and/or iron alloys aremelted in an electric arc furnace containing a bath of molten steel anda slag layer, wherein the slag composition contains MgO in an amount of8 to 20% by weight, and has a basicity of from about 1.5 to about 2.0.

According to a preferred embodiment of the invention, the slagcomposition contains MgO in an amount of about 10 to 17% by weight, andmore preferably about 13 to 15% by weight.

In another aspect, the present invention provides a process for makingsteel in an electric furnace having a plurality of electrodes, theprocess comprising: maintaining a bath of molten steel in the furnace,the bath being maintained in a liquid condition by heat supplied fromthe electrodes; maintaining a layer of molten slag covering the bath ofmolten steel, wherein the slag has a melting point which issubstantially the same as or greater than a temperature of the steel inthe bath; charging a charge material into the furnace, the chargematerial comprising iron and/or an iron alloy; melting the chargematerial in the furnace; and withdrawing the molten steel from the bath.

Preferably, the molten steel in the bath has a carbon content of lessthan or equal to about 0.10% by weight; with the charge materialcomprising direct reduced iron, and optionally including steel in anamount of up to about 20% by weight. The charge material is preferablycharged into the furnace continuously, with the bath of molten steelpreferably having a volume of from about 7 to about 13 tap volumes.

Preferably, heat is transferred from the electrodes to the metal bath byopen arcs, with the power density in the furnace hearth being from about250 to about 500 kW/m². The slag is preferably superheated by about 40to about 150° C. above the bath temperature,

In yet another aspect, the present invention provides a plant formanufacturing steel, comprising: a direct reduction furnace forproducing DRI; and a stationary electric arc furnace for melting theDRI, the electric arc furnace having a plurality of electrodes fromwhich heat is supplied for melting the DRI; wherein the DRI is fed bygravity from the direct reduction furnace to the electric arc furnace.

Preferably, one or more DRI conduits extend between the direct reductionfurnace and the electric arc furnace for transferring DRI from thedirect reduction furnace to the electric arc furnace. Each DRI conduitcommunicates with the interior of the electric arc furnace through a DRIcharging port located in the furnace roof, and the conduits arepreferably inclined at an angle of 45 to 90° to the horizontal. Each DRIcharging port is preferably located adjacent to a pair of electrodes,such that the electrodes and the charging port form a triangle, with thecharging port located at an apex of the triangle which is substantiallyequally spaced from each of the electrodes.

In addition, one or more scrap conduits preferably extend between thedirect reduction furnace and the electric arc furnace for charging steelscrap into the electric arc furnace. Each scrap conduit communicateswith the interior of the electric arc furnace through a scrap chargingport located in a roof of the electric arc furnace. Preferably, eachscrap charging port is located between adjacent pairs of electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic plan view of a stationary six electrode arcfurnace adapted for use in the continuous steelmaking process accordingto the invention;

FIG. 2 is a schematic side elevation of a preferred plant layoutaccording to the invention, and includes the stationary six electrodearc furnace shown in FIG. 1;

FIG. 3 is a graph of FeO content versus CaO/SiO₂ ratio; and

FIG. 4 is a ternary FeO—CaO—SiO₂ diagram from which the melting point ofslag can be estimated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The preferred embodiments of the invention will now be described belowwith reference to the use of a stationary six-electrode arc furnace 110(also referred to herein as a “steelmaking furnace”) as a continuousmelter. The use of this type of furnace is well established in thesmelting of nickel and copper. Despite the references to this type offurnace in the prior art mentioned above, it is not typically used inthe steel industry.

As shown in FIG. 1, the furnace 10 is preferably rectangular in shape,having side walls 12, 14 and end walls 16, 18. The furnace walls andhearth are lined with refractory brick (not shown) which is based onmagnesium oxide. In a particularly preferred embodiment of theinvention, the furnace may have dimensions of about 9×27 metres,sufficient to provide a relatively large bath size of about 7 to 13 tapvolumes, more preferably about 10 tap volumes. The large bath provides a“flywheel” effect, enabling the periodic tapping of steel whilecontinuing to melt the charge material, and also ensures sufficientanalytical compensation of the steel chemistry and sufficient heat forthe melting process.

The preferred embodiment of the invention will be described inconnection with the use of direct reduced iron (DRI) as a feed material.DRI is produced by a direct reduction shaft furnace (FIG. 2) upstreamfrom the steelmaking furnace 10. The DRI is preferably fed to thesteelmaking furnace 10 in the form of briquets or spheres having adiameter of about 12 to 16 mm. The DRI may either be hot or cold, but ispreferably hot. The typical chemical composition of DRI produced in ashaft furnace is as follows (as percentages): Fe(total)=92.34;Fe(metallic)=85.22; FeO=9.54; C=1.71; SiO₂=1.51; Al₂O₃=0.8; CaO=0.72;MgO=0.28; S=0.01; P=0.04; other residuals=0.19.

The steelmaking furnace is preferably operated in open-bath mode,utilizing the natural slag-foaming characteristics associated with thecontinuous feed/melting of DRI. The slag composition is discussed ingreater detail below. The walls and roof of the furnace are cooled bystaves and finger coolers (as described in U.S. Pat. No. 5,378,260,incorporated herein by reference). The cooling of the walls causessolidification of slag around the perimeter of the bath. This slag“accretion” protects the refractory lining of the furnace in the slagarea and at the slag/metal interface.

Electric arc power is delivered to the furnace 10 from threesingle-phase furnace transformers 20, each coupled to a pair ofelectrodes 22 which are immersed in the slag layer in the furnace 10.The total furnace power is preferably at least about 100 to 120MW, withthe hearth area preferably having a power density of from about 250 toabout 500 kW/m². This power density is an order of magnitude lower thanthat of a conventional steelmaking electric arc furnace, which has apower density of about 2000 to 2500 kW/m². Conventional furnaces requirea higher power density so that batchwise melting of iron/steel can becarried out as quickly as possible. The lower power density of thefurnace used in the present invention is a major factor contributing tothe extended refractory life, as it reduces radiation heat flux to therefractory lining and prevents excessive slag superheating, discussedmore fully below. The steel and slag are periodically tapped from tapholes 24 and 26, respectively, which are preferably located on oppositeends of the furnace. The tap holes 24, 26 are closed by “mud” from a mudgun, and are opened either by drilling or burning the mud from the tapholes.

In a preferred embodiment of the invention, the steel making furnacedescribed above is coupled to a direct reduction shaft furnace. In thepreferred plant layout shown in FIG. 2, the direct reduction shaftfurnace 28 is “stacked” above the steelmaking furnace 10 with the outletof the shaft furnace being spaced vertically above the charging ports ofthe steelmaking furnace, so that the DRI can be fed essentially bygravity directly from the direct reduction shaft furnace 28 to thesteelmaking furnace 10. Although the DRI is fed by gravity in thepreferred embodiment of the invention, it will be appreciated that theDRI may instead be delivered to the steelmaking furnace 10 pneumaticallyor by mechanical conveyors. Screw conveyors are used to transfer the hotDRI from the shaft furnace 28 to hoppers from which it is conveyed tothe steelmaking furnace 10.

In the preferred plant layout shown in FIG. 2, the DRI is gravity-fed tothe steelmaking furnace 10, with the output of the shaft furnace 28being delivered to furnace hoppers 30, 32, 34 from where it is deliveredto furnace 10 through runners 36, 38, 40. The DRI is preferably fedthrough runners 36, 38, 40 in the form of spherical pellets as describedabove. In order to prevent blockage of the runners, the inventors havefound that the runners are preferably inclined at an angle of from about45 to 90° to the horizontal.

The runners deliver the DRI to the furnace 10 through a plurality of DRIcharging ports 42, 44, 46 located in the furnace roof 48. The DRIcharging ports are located so as to provide a symmetrical distributionof charge materials in the furnace 10 and to prevent damage to theelectrodes. For optimum distribution of DRI, the inventors have foundthat each DRI charging port should be located in the furnace roofadjacent to a pair of electrodes so that the electrodes and the chargingport form a triangle, with the apex of the triangle at which thecharging port is located being substantially equally spaced from eachelectrode.

The charge material fed to the steelmaking furnace in the presentinvention may preferably include scrap steel in amounts of up to 20% byweight. This scrap steel is preferably preheated and is sufficientlyreduced in size to allow it to be continuously fed by a conveyor 48 tohoppers 50, 52, from where it is delivered to furnace through runners54, 56. The scrap enters the furnace 10 through two scrap charging ports58, 60 located in the furnace roof, each scrap charging port preferablybeing located between two pairs of electrodes 22 for optimumdistribution. The use of this scrap charging system eliminates the needfor a high capacity furnace charge crane.

As the DRI and scrap steel are continuously charged and melted in thesteelmaking furnace 10, the tap holes are opened periodically to tap thesteel and slag. The semi-finished steel is transferred in ladles 62 on acontinuous basis to a ladle metallurgical furnace 64 for refining andfinishing, from which the steel ladles are brought to a continuous slabcaster 66.

A set of specific metallurgical process requirements are required tosupport stable continuous operation of the steel making furnaceaccording to the invention. For example, the carbon content of theinitially charged DRI should be reduced to less than 0.10%, and morepreferably to less than 0.05%, in the steelmaking furnace. This ispreferably achieved Without injection of gaseous oxygen. Furthermore,the chemical and physical properties of the slag must be engineered tosupport the slag foaming which is necessary in open-bath melting ofcontinuously fed DRI, and to avoid slag-line refractory erosion.

These requirements which must be met by the slag are somewhatconflicting. For example, carbon content of the steel is reduced byincreasing FeO content in the slag. However, FeO reacts with therefractory materials, which are comprised primarily of MgO. In order toproduce a slag which meets these requirements, the inventors have foundthat it is particularly advantageous to provide a slag composition whichhas a melting point substantially the same as or greater than that ofthe steel bath temperature, so as to avoid excessive superheating of theslag. As used herein in relation to the slag melting point,“substantially the same” means±30° C. relative to the steel bathtemperature. During operation of the steelmaking furnace, the slag issuperheated above its melting point and above the melting point of thesteel in the bath. In the present invention, the slag is preferablysuperheated by about 40 to about 150° C. above the slag melting pointand the steel bath temperature. Having a slag melting point close to thesteel bath temperature means that the slag will be superheated to alesser degree than if a lower melting slag was used as, for example, inthe above-mentioned Rath patent. As the fluidity of the slag isproportional to the degree of superheating, the slag according to thepresent invention will be relatively less fluid (i.e. more viscous) thana highly superheated slag, such as that disclosed by Rath. This lowerfluidity translates to reduced erosion of the furnace refractory.

According to the invention, achieving a higher slag melting temperatureis at least partially accomplished by increasing the MgO content in theslag. Increasing the MgO content also directly reduces the rate oferosion of the refractory walls, independent of its effect on the slagmelting temperature. According to the present invention, the MgO contentof the slag is preferably in the range of from 8 to 20%, more preferablyfrom about 10 to 17%, and even more preferably from about 13 to 15%.

To further prevent refractory erosion, the slag preferably has arelatively low CaO/SiO₂ ratio, thereby reducing the slag basicity andreducing the amount of FeO required in the slag for a given carboncontent. In a particularly preferred embodiment, the CaO/SiO₂ ratio(also referred to herein as “basicity”) is preferably from about 1.5 toabout 2.0. FIG. 3 is a graph which shows the relationship between FeOcontent and CaO/SiO₂ ratio.

Manipulation of the chemical composition of the slag can be at leastpartially accomplished by adjusting the composition of the DRI. Forexample, the current level of process control of gas-based directreduction processes allows the metallization (i.e. the amount ofresidual iron oxides) and the carbon content of the DRI to be setindependently. Thus, the “natural” melt-in carbon of the steel bath canbe targeted through the DRI composition, while maintaining sufficientfoaming of the slag. Secondly, the DRI typically contains low levels ofsulphur and phosphorus. Therefore, removal of these elements in thesteelmaking furnace is not a major requirement, providing greaterflexibility in selecting the slag composition.

Depending on the composition of the DRI, it may also be preferred to addamounts of CaO, SiO₂ and MgO to the slag. This may be accomplished byadding amounts of high-calcium lime and/ordolomitic lime to the furnace.High-calcium lime/dolomitic lime typically have the following chemicalcompositions (as percentages): CaO=96/56.6; MgO=1/40.6; SiO₂=1/0.8;Fe₂O₃=0.2/0.2; Fe=0.14/0.14; Al₂O₃=0.5/0.4; L.O.I.=1.3/1.4.

As an example, Table 1 illustrates calculated slag compositions for theproduction of steel having carbon contents of 0.10% and 0.06%.

TABLE 1 Calculated Stag Components as a Function of Carbon in Steel SlagComponent 0.06% C Content 0.10% C Content SiO₂ 18.0 22.6 CaO 27.5 34.7MgO 9.7 10.0 Al₂O₃ 9.5 12.0 FeO 32.4 17.4 Other 2.9 3.3 CaO/SiO₂ 1.5 1.5

FIG. 4 comprises a ternary FeO—CaO—SiO₂ diagram which can be used toestimate the melting points of slag compositions according to theinvention. In order to estimate the melting point of a slag composition,it is assumed that CaO is the sum of CaO+MgO and SiO₂ is the sum ofSiO₂+Al₂O₃. While recognizing that real world multi-component slagsystems are far more complex, the approximate melting point provided bythis method serves as a useful guide. Using the diagram of FIG. 4, theprojected melting point for slag in the production of steel according tothe invention with 0.06% carbon is approximately 1500-1530° C. This issubstantially the same as the temperature of the steel bath which, inthe case of steel having a carbon content of 0.06%, is about 1530° C.

In addition to lengthening furnace campaign life, the present inventionis expected to provide other advantages. For example, the preferredplant configuration described above provides capital cost savings due tosimplified scrap handling, less extensive DRI handling circuits, lowerbuilding costs and less expensive arc furnace. A plant according to thepresent invention may also have lower operating costs than aconventional plant, due in part to less refractory erosion, lowermaintenance costs and lower dust disposal costs. Furthermore, thestationary electrode furnace is better matched than conventional arcfurnaces to the direct reduction shaft furnace in terms of annualoperating time. Specifically, the annual availability of both the shaftfurnace and the stationary arc furnace are about 8,000 hours, whereasthat of a conventional steelmaking furnace is about 7,200 hours.

Another advantage of the present invention, and one which distinguishesit from the Rath and Gudenau et al. patents mentioned above, is the useof open arcs, formed between the electrodes and the metal bath in thearc furnace, to provide the radiation heat for chemical reactions andfor melting the charge materials. In the processes disclosed by Rath andGudenau et al., arcing is preferably avoided and the electric energy ofthe electrodes is transformed to Joule heat by resistance heating of theslag. Resistive heating provides relatively low heat generation,resulting in low furnace productivity.

The formation of open arcs in the process of the invention isaccomplished by the use of a relatively higher voltage than that used inthe Rath and Gudenau et al. patents. Since the electrodes are immersedin the foaming slag in the process of the present invention, someportion of the heat will be provided by transformation of the electricalenergy into Joule heat by electrical resistance of the slag, therebykeeping the bath temperature constant and providing additional heat forthe melting process. The slag electrical conductivity for steel having acarbon content of 0.3% by weight is about 0.35-0.45 om⁻¹cm⁻¹, which isthree times lower than that for steel having a carbon content of 0.06%by weight (1.2-1.3 om⁻¹cm⁻¹), with the slag resistance varying in thereverse order. The power released in the bath can be expressed by thefollowing formula, which is disclosed in U.S. Pat. No. 3,715,200,incorporated herein by reference:P _(bath) =P _(arc) /[V _(t) ²/(P _(t) R _(bath))−1]wherein P_(bath) is bath power; P_(arc) is arc power (a function of arclength); V_(t) is phase voltage; P_(t) is phase power; and R_(bath) isbath resistance.

Therefore, to maintain the same power release in the bath and the samebath temperature it is necessary to increase the voltage by a power oftwo, or to increase the voltage by some extent with a simultaneousdecrease of arc power or arc length.

Although the invention has been described with reference to certainpreferred embodiments, it is not intended to be limited thereto. Rather,the invention includes within its scope all embodiments which may fallwithin the scope of the following claims.

1. A process for making steel in an electric furnace having a pluralityof electrodes, the process comprising: (a) maintaining a bath of moltensteel in the furnace, said bath being maintained in a liquid conditionby heat supplied from said electrodes; (b) maintaining a layer of moltenfoaming slag covering the bath of molten steel, wherein a melting pointof the slag is substantially the same as or greater than a melting pointof the steel in the bath, wherein the slag comprises SiO₂, CaO, MgO,Al₂O₃ and FeO and is substantially free of manganese, the MgO content ofthe slag is about 8 to 20 percent by weight and the ratio of CaO/SiO₂ inthe slag is about 1.5 to 2.0; (c) charging a charge material into thefurnace, said charge material comprising iron and/or an iron alloy; (d)melting the charge material in the furnace by use of open arcs betweenthe electrodes and the bath; and (e) withdrawing the molten steel fromthe bath.
 2. The process according to claim 1, wherein the molten steelhas a carbon content of less than or equal to about 0.10% by weight. 3.The process according to claim 1, wherein the slag is superheated byabout 40 to about 150° C. above a temperature of the bath.
 4. Theprocess according to claim 1, wherein the charge material comprisesdirect reduced iron.
 5. The process according to claim 1, wherein thecharge material includes scrap steel in an amount of up to about 20% byweight.
 6. The process according to claim 1, wherein a power density ina hearth of the furnace is from about 250 to about 500 kW/m².
 7. Theprocess according to claim 1, wherein the bath has a volume of 7 to 13tap volumes.
 8. The process according to claim 1, wherein the chargematerial is continuously charged into the furnace.
 9. The processaccording to claim 1, wherein the slag contains MgO in an amount ofabout 10 to 17% by weight.
 10. The process according to claim 1, whereinthe slag contains MgO in an amount of about 13 to 15% by weight.